IN THIS ARTICLE Calculate Pot Volume | How To Use Potting Soil | What Size Bags | Our Picks
How much potting soil mix do I need for my container garden? This is one of those questions I ask myself every spring, and every spring I follow the same process to work it it out. If you use standard container/pot sizes then the calculation is simple. For special containers/pots, then I use an online calculator to make it easy.
There are two easy methods you can use to work out the Potting Soil you need. Use our plant pot volume table or use an online tool. That simple.
It really is a very simple process to work out how much potting soil mix you need. I’ll take you through the process I follow below.
TIP We recommend an All-purpose Potting Soil like the best selling product below.
How to Calculate Potting Soil Volume for Standard Pots
There is a standard range of plant pots available that have standard sizes. These are round and have set dimensions. I’ve put together a list of standard container/pot sizes and the estimated amount of Potting Soil Mix needed to fill each size. Each container/pot is measured from the inside so we’re using internal measurements to estimate the volume.
Table of Volume for Standard Round Containers or Pots
4-inch (10 cm) pots use about 0.5 quart (.5 Liter) of potting soil.
6-inch (15 cm) pots use about 1.5 quarts (1.5 Liter) of potting soil.
8-inch (20 cm) pots use about 4 quarts (4 Liters) of potting soil.
10-inch (25 cm) pots use around 7 quarts (7 Liters) of potting soil.
12-inch (30 cm) pots use about 12 quarts (11 Liters) of potting soil.
14-inch (35 cm) pots use about 18 quarts (17 Liters) of potting soil.
How to Calculate the Volume of a Square or Rectangular Pot
If you have square or rectangular containers/pots, I recommend using the online volume tool detailed in the next paragraph. It’s so quick and easy. However, if you want to calculate the volume by hand, then use the following steps.
- Measure the internal Length, Width, and Height in Centimeters as this can be converted easily to Quarts, Gallons, or Liters.
- Multiply Length by Width by Height to get your total volume in Centimeters. For example 10 cm * 10 cm * 10 cm = 1000 cm3
- Using Google, you can convert any measure of volume to any other. For example 1000 cm3 is equal to 1.06 US Quarts.
Note the total volume of each container or pot you have measured. Count the number of containers/pots you have of each size, then multiply the volume for that size by the number of containers/pots you have. For example 4 times 8-inch pots equals 16 quarts (16 Liters) of Potting Soil is needed. Finally sum your total volumes.
TIP Always add a little extra volume above your Potting Soil estimate. It allows for spillage.
How to Calculate Potting Soil Volume for Non-standard Pots
The method I use for non-standard pots is very simple, use an on-line calculator. There are a number of great tools out there and this is the one I use “Omni Calculator“. It’s completely free, no login needed, and simple to use.
Before you click on the link to the calculator, you need to measure the inside Height and inside Diameter of you container/pot if it’s circular. Square or rectangular containers you need inside Length, Width, and Height. It must be the inside measurement as this is the space we plan to fill with Potting Soil. The calculator lets you set the shape you need.
You also need to choose the unit of measure you want to use. You can use inches or centimeters, which ever you’re comfortable with. Then you can choose the unit of measure it will use to return the volume of Potting Soil, for example quarts, gallons, liters. It’s all there.
At the end, you will have a volume of Potting Soil you need to fill your container or pot.
How To Use Potting Soil Mix
Unlike garden soil, Potting Soil is an all-in-one plant growing medium, so nothing needs to be added to improve it. All purpose Potting Soil Mixes will usually be sterilized (heat-treated) to eliminate all living organisms, seeds, insects, and plant diseases. They are made by blending compost, peat, humus, and pumice. Each brand has their own recipes and varieties, some are even made for specific plant types.
TIP Always clean your container or pot before use. It helps prevent unwanted bacteria in your potting soil. You can use simple household detergent and a firm bristle brush.
Take a clean pot and your seeding plant with a full root-ball. In the bottom of your new container/pot, place a small amount of potting soil. Add enough Potting Soil so that the top of your root-ball will be about half an inch (1.5 centimeters) from the inside top of the pot (once the plant is seated inside). Place the seedling in the middle of you container/pot and scoop in Potting Soil filling in the space around the sides of the root-ball, until it is just covered. Firm this down by hand, then top up as needed. Finally, thoroughly water the Potting Soil to help the plant settle into its new home.
TIP Always wear Garden Gloves when handling Potting Soil, this helps to protect you from bacteria that can live in the soil mix.
What Size Potting Soil Bags Should I Get
Depending on where you live, Potting Soil comes in bags measured in Quarts/Cubic Feet or Liters. In the USA common sizes are 4, 8, and 16 Quarts (roughly 4,8, and 16 Liters) then 1 and 2 Cubic Feet (30 or 60 Liters).
Once you have added up the total volume of Potting Soil Mix needed, search for brand and volume required. I also round my volume up, better to have too much than not enough. We are dealing with estimates after all.
Once last thing to consider is where will you store your Potting Soil Mix bags, the unused portion? If space is an issue, then choose a bag size that will easily fit into the storage space you have available. Don’t forget, the larger the bag, the heavier the bag. Smaller bags are not only easier to store, they are easier to handle!
TIP I recommend you buy 1 extra bag of Potting Soil Mix, enough to top up your containers/pots after the Potting Soil has settled.
Potting Soil Mix is a wonderful thing. It’s beautiful clean rich soil ready to use and ensure you have a healthy plant. We’ve outlined two techniques you can use to help you estimate just how much Potting Soil you’re likely to need. You want to fill your containers or pots, and have a little left over. Just a little. I hope this article has given you the information you need.
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